How, precisely, does the water of the brain turn into the wine of consciousness? Pursuing this question has been a growing preoccupation of neuroscientists; the answer will have implications for the diagnosis and treatment of brain disorders such as coma and other unresponsive states. One obstacle to this quest has been the lack of experimental means to uncover the basic neural “core” or minimal brain requirements of consciousness. Now, Långsjö and colleagues have used general anesthetics as a tool to probe the brain for insight into our inner life.

Rather than study people “going under” general anesthesia, as has often been done, the investigators examined the “coming out” phase of dexmedetomidine- and propofol-induced unconsciousness. This approach enabled the identification of the neural correlates associated with the most primary form of consciousness— recovery from the oblivion of anesthesia. While they were monitored by positron emission tomography (PET), human volunteers emerged from anesthesia into consciousness, as assessed by movement in response to a verbal command. Consciousness was associated with activation of subcortical areas that mediate arousal (midline thalamus, hypothalamus, and locus ceruleus) and key higher processing areas (anterior cingulate and frontal and parietal cortices). As participants became conscious, the authors noted a particular increase in the connections between the frontal and parietal lobes. Of note, the activated cortical areas were limited in scope, suggesting that extended cortical involvement is not required for the most basic form of consciousness.

There is a caveat, however. The verbal command to identify consciousness was “Open your eyes.” Many auditory stimuli may result in eye opening in arousable (but not necessarily conscious) individuals, so further work should investigate more definitive commands (such as “Squeeze my hand twice.”). Response to this command may require more extensive participation of the neocortex. Nevertheless, this study has identified the neural core of the most fundamental form of consciousness. Långsjö and colleagues offer important results that could guide monitoring during general anesthesia and the behavioral recovery of patients after coma or vegetative states.